1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine or a printer which forms an image on a recording material through an electrophotographic process to obtain a hard copy.
2. Related Background Art
Up to now, there have been proposed various image forming apparatuses each of which includes a plurality of image forming portions, forms toner images different in color from each other in the respective image forming portions, sequentially superimposes and transfers the toner images onto the same recording material, to thereby form a color image on the recording material. Under the circumstances, a color copying machine of the multi-color electrophotographic system using an endless recording material bearing member has been employed for high-speed recording.
In the color image forming apparatus of this type, image forming portions of four colors consisting of yellow, magenta, cyan and black are disposed along a transfer belt which is a belt-like recording material bearing member. Primary charging, image exposing and developing processes are conducted on photosensitive drums disposed in the respective image forming portions as image bearing members to form toner images of the respective colors. The toner images are superimposed and transferred onto a recording material which is attracted on the transfer belt and sequentially conveyed to transfer portions which are disposed opposite to the photosensitive drum of the respective image forming portions. Then, after the recording material onto which the toner images of four colors have been transferred is separated from the transfer belt, the toner image is fixed onto the transfer material to obtain a fixed image of full colors on the recording material.
The transfer belt is formed of a sheet made of dielectric resin such as polyethylene terephthalate resin (PET), polyvinylidene fluoride resin or polyurethane resins, and a belt in which both end portions of the sheet are permitted to lie one upon another and bonded into an endless shape or a seamless belt is used.
The recording material is extracted from a recording material cassette and then supplied onto the transfer belt through a plurality of conveying rollers and registration rollers, and then electrostatically attracted onto the transfer belt due to an attraction bias applied to an attraction charging means.
Note that the attraction charging means for the recording material as used is a non-contact charger such as a corona discharger or a contact charger using a charging member such as an electrically conductive blade, roller or brush.
For example, in the case where the attraction charging means is formed of the electrically conductive roller (attraction roller), after the recording material has been supplied to the transfer belt, the recording material is nipped together with the transfer belt between upper and lower attraction rollers that face each other through the transfer belt in the attraction portion, and the recording material is electrostatically attracted due to the action of the attraction bias applied to one of the attraction rollers.
However, when a small-sized recording material is going to be attracted, a current that flows due to the attraction bias is escaped to portions other than the recording material, charges that contributes to the attraction of the recording material onto the transfer belt decreases and the attraction of the recording material becomes insufficient, to thereby lead to a problem such as the recording material conveyance failure or the transfer failure.
The present invention has been made under the above circumstances, and therefore an object of the present invention is to provide an image forming apparatus that conveys a recording material stably while electrostatically attracting the recording material onto a recording material bearing member regardless of a size of the recording material so as to excellently form an image on the recording material without having an attraction failure or a transfer failure.
In order to achieve the above object, according to the present invention, there is provided an image forming apparatus comprising:
image forming means for forming an image on a recording material;
a recording material bearing member for bearing and conveying the recording material toward said image forming means;
an attraction charging member that is disposed so as to come into contact with said recording material bearing member and gives charges to electrostatically attract the recording material onto said recording material bearing member;
a power supply for supplying an electric current to said attraction charging member; and
control means for changing a value of the electric current in accordance with the length of the recording material in a direction perpendicular to a recording material conveying direction.